Splitting Core Systems/Infrastructure (SystemsRx) from EcsRx

Summary

Currently a lot of the EcsRx features are not directly tied to the ECS paradigm but as they are part of core they come bundled together.

Part of this change is to decouple some of that so there can be a SystemsRx library which has no notion of EcsRx which can be used by people who want to have nice infrastructure and DI abstractions, as well as nice simple systems to run logic in a simple way, but don't want to have to worry about using entities and components.

SystemsRx

So this split has now made it so the new library acts as a layer underneath EcsRx and provides basic System Execution, Infrastructure, DI and common extensions as well as a couple of simple systems such as IManualSystem and event handling systems, it even supports plugins without needing to worry about the EcsRx layer.

EcsRx

At the high level EcsRx hasn't changed much, its just a lot of the core innards have been moved down a layer (and this is mentioned more on front page readme), there are a couple of breaking changes (mentioned in breaking changes document), but for the most part you continue to use the library as you do right now.

Under the hood to use EcsRx you will be pulling in SystemsRx but this simplifies things a bit and also lets you run systems outside of the ECS paradigm alongside your ECS related ones, so its best of both worlds.